1. Field of the Invention
This invention relates to an electrophotographic photoconductor having an inorganic filler-containing photoconductive layer, a method of manufacturing same and to an image forming apparatus using same. More specifically, the present invention is directed to an electrophotographic photoconductor having a long service life, to a method of manufacturing same and to an image forming method, an image forming apparatus and a process cartridge using same. The image forming apparatus and process cartridge are utilized in, for example, electrophotographic copying machines, facsimile apparatuses, laser printers and direct digital printing master making apparatuses.
2. Discussion of the Related Art
The electrophotographic process using an electrophotographic photoconductor includes at least the steps of conducting first charging for uniformly charging the surface of the photoconductor, exposing the charged surface of the photoconductor to light images to form latent electrostatic images thereon, developing the latent electrostatic images with toner to make visible toner images, transferring the toner images to a transfer sheet, fixing the toner images to the transfer sheet, and cleaning the surface of the photoconductor.
Electrophotographic photoconductors used in the above electrophotographic process are desired to have the following properties:
(1) good charging property so as to be charged to an appropriate electric potential in a dark place;
(2) good charge maintaining property such that the decrease of the electric potential is little in a dark place;
(3) good charge dissipating property such that the electric potential is rapidly dissipated by light irradiation;
(4) capability of being produced with relatively low costs;
(5) adaptability to minimize environmental pollution; and
(6) capability of producing good images without image defects such as background fouling for a long time.
Conventional photoconductive layers for use in the photoconductors include selenium photoconductive layers of selenium or a selenium alloy supported on a conductive support; inorganic photoconductive layers containing a binder and an inorganic photoconductive material such as zinc oxide or cadmium sulfide dispersed in the binder; amorphous silicon photoconductive layers of an amorphous silicon material; and organic photoconductive layers containing an organic photoconductive material. In photoconductors for use with the electrophotographic method, organic photoconductive materials are now widely used because such organic photoconductors can be manufactured at low costs by mass production and will not cause environmental pollution.
Many kinds of organic photoconductors are conventionally proposed, for example, a photoconductor employing a photoconductive resin such as polyvinylcarbazole (PVK); a photoconductor comprising a charge transport complex of polyvinylcarbazole (PVK) and 2,4,7-trinitrofluorenone (TNF); a photoconductor of a pigment dispersed type in which a phthalocyanine pigment is dispersed in a binder resin; and a function-separating photoconductor comprising a charge generation material and a charge transport material. In particular, the function separating photoconductor has now attracted considerable attention.
The mechanism of formation of an electrostatic latent image using the function-separating photoconductor is considered to be as follows:
(1) upon irradiation of a charged organic photoconductor with light, the light passes through a transparent charge transporting layer and is absorbed by a charge generating material contained in a charge generating layer;
(2) the charge generating material which has absorbed the light generates a charge carrier;
(3) the charge carrier, which is injected to the charge transporting layer, moves through the charge transporting layer, which is caused by the electric field formed in the charged photoconductor; and
(4) the charge carrier finally combines with the charge on the surface of the photoconductor, resulting in neutralization of the charge, and thereby an electrostatic latent image is formed.
Functionally separated photoconductors which include a combination of a charge transporting material which has absorbance mainly in an ultraviolet region and a charge generating material which has absorbance mainly in a visible region are well known and preferable. However, even in the functionally separated photoconductors, the durability is not necessarily satisfactory.
Among various image forming machines, electrophotographic apparatuses are now widely distributed for use in offices as well as for domestic, personal use because of their high speed recording. In line with such a trend, there are increasing demands for small-sized machines and running trouble-free machines. In particular, there are increasing demands for machines which can reduce running costs, which permit high-speed printing and which are capable of producing color images. In connection with color printing, production of high grade images of natural and clean figure and landscape are strongly desired.
To respond to such demands, charge rollers are increasingly used in lieu of a scolotron chargers so as to reduce electric power consumption and generation ozone. Further, many attempts are made to use chargers with means for superimposing AC components for the purpose of stabilizing the image quality. Improvement of image quality by using small particle size developer is also proposed in both monochromatic and color printing or copying machines. In view of the fact that a dye or pigment for printing ink has a size of sub-micron order, there still remains an objective problem to develop a toner having a much reduced size. In terms of small-sized and high speed printing and copying machines, electrophotographic photoconductor must be used at high speed. Such machines pose increased hazard to electrophotographic photoconductors. Thus, it is one of the greatest problems to develop an electrophotographic photoconductor having excellent durability.
In order to always obtain stable output images throughout a large number of printing operations, development of techniques for preventing image defects, reduction of image density and reduction of resolution is essential. Such image defects are known to result from scars or scraping of a surface top layer of the photoconductor. Thus, in order to prevent occurrence of image defects during a large number of printing operations, it is necessary that the organic type electrophotographic photoconductors should have high mechanical strengths and excellent abrasion resistance, while ensuring suitable electrostatic characteristics.
Various proposals have been made to improve the abrasion resistance of the surface of the photoconductors are as follows:
(1) Improving Mechanical Strength of Charge Transporting Layer:
For example, Japanese Laid-Open Patent Publications Nos. 10-288846 and 10-239870 disclose photoconductors in which the abrasion resistance thereof is improved by using a polyacrylate resin as a binder resin. Japanese Laid-Open Patent Publications Nos. 9-160264 and 10-239871 disclose photoconductors in which the abrasion resistance thereof is improved by using a polycarbonate resin as a binder resin. Japanese Laid-Open Patent Publications Nos. 10-186688, 10-186687, and 5-040358 disclose photoconductors in which the abrasion resistance thereof is improved by using a polyester resin having a terphenyl skeleton, a polyester resin having a triphenyl methane skeleton, or a polyester resin having a fluorene skeleton as a binder resin. Japanese Laid-Open Patent Publications Nos. 9-12637 and 9-235442 disclose the use of a polymer blend containing a styrene elastomer as a binder for a charge transporting layer.
With the photoconductor mentioned above, however, it is necessary to use a large amount of a charge transporting material having low molecular weight in the photoconductive layer in order to obtain good light decaying property, i.e., good photosensitivity. To use a large amount of a charge transporting material having low molecular weight seriously deteriorates the strength of the photoconductive layer, and the more the amount of the charge transporting material in the photoconductive layer, the worse becomes the abrasion resistance of the photoconductive layer. Therefore the photoconductive layers of the above photoconductors easily abrade due to the charge transporting material having low molecular weight. Accordingly the use of a specific binder for a charge transporting layer is not effective for the improvement of abrasion resistance of photoconductors.
(2) Using Charge Transporting Polymer Material:
Japanese Laid-Open Patent Publication No. 7-325409 discloses a photoconductor which includes a charge transporting polymer material instead of charge transporting materials having low molecular weight. It is supposed that the photoconductor has good abrasion resistance because the content of resins in the photoconductive layer is relatively high.
However, a mere use of a charge transporting polymer material in place of a low molecular weight charge transporting material is not always sufficient to impart satisfactory printing resistance to the photoconductor. One possible reason is that abrasion of the photoconductor is not only attributed to mechanical load applied thereto but also ascribed to deterioration of surfaces thereof due to electric shock or chemical attack by oxidizing substances such as ozone. For example, when AC superposition charging is adopted to obtain uniform charging, surfaces of the photoconductor are subjected to repeated bombardment of charges corresponding to the frequency of the AC voltage, which would cause a reduction of printing resistance thereof. Additionally, because it is not easy to obtain a highly pure charge transporting polymer material, impurities are apt to be contained therein, which is likely to cause accumulation of residual potential.
(3) Decreasing Friction Coefficient of Charge Transporting Layer:
For example, Japanese Laid-Open Patent Publications Nos. 10-246978 and 10-20534 disclose photoconductors which have a relatively low friction coefficient by including a lubricant such as siloxane. Japanese Laid-Open Patent Publications Nos. 5-265241 and 8-328286 disclose photoconductors which have a relatively low friction coefficient by including a particulate fluorine containing resin. A reduction of the friction coefficient of a photoconductor may reduce a contact pressure between the photoconductor and a transfer medium, etc., so that the durability of the photoconductor will be improved. However, the lubricant generally is not compatible with a binder of the charge transporting layer and is apt to appear on the surface of the layer. As a result, the lubricant is gradually lost during use to cause the lowering of the abrasion resistance. A lubricant having good compatibility with the binder, on the other hand, is generally small in friction coefficient.
(4) Providing Protective Layer
For example, Japanese Laid-Open Patent Publications Nos. 57-30846, 58-121044, 59-2234443 and 59-223445 disclose a photoconductor having a protective layer containing antimony oxide or tin oxide having specific particle size and particle size distribution. While the use of such a protective layer can improve the mechanical strengths of the photoconductor and durability thereof, the resolution of the photoconductor tend to be lowered.
In particular, such a reduction of the resolution occurs when ions generated by a charging device deposit on the surface of the photoconductor. Probably, the deposition of ions causes leakage of charges in the direction parallel with the surface of the photoconductor, which in turn results in the lowering of the resolution. In the case of a photoconductor which can completely resist against surface wearing, fouling substances are apt to accumulated thereon upon repeated use, so that the electric resistance of the surface of the photoconductor gradually decreases. Such a phenomenon often occurs with photoconductors having a surface protective layer.
It is not easy to control the rate of wear of a surface protective layer. Further, the thickness of the protective layer should be thin since otherwise the residual potential increases. In addition, a small size photoconductor drum for use in a small size electrophotoconductive machine is apt to cause delamination of its protective layer having a small radius of curvature. Thus, the use of a surface protective layer poses a number of problems and, therefore, is not practically applicable.
(5) Modifying Charge Transporting Layer:
For example, Japanese Laid-Open Patent Publications Nos. 46-782 and 52-2531 disclose photoconductors in which a lubricating filler is incorporated in a surface layer thereof to improve the service life thereof. Japanese Laid-Open Patent Publications Nos. 54-44526 and 60-57346 disclose photoconductors in which a filler is incorporated in an insulating layer of an image-holding member or a photoconductive layer to improve the mechanical strengths thereof. Japanese Laid-Open Patent Publications Nos. 1-205171 and 7-261417 disclose photoconductors in which a filler is incorporated in a charge transporting layer or a surface layer thereof to enhance the hardness thereof and to impart slipping properties thereto. Japanese Laid-Open Patent Publication No. 61-251860 discloses a photoconductors in which 1-30 parts by weight of hydrophobic titanium oxide powder is used per 100 parts of a charge transporting medium to improve the mechanical strengths thereof.
These methods, however, cause accumulation of residual potential and deterioration of sensitivity. Namely, known photoconductors having a filler-containing photoconductive layer cause considerable increase of the residual potential when the thickness thereof increases.
In the case of a photoconductor whose surface wearing is suppressed by improvement of the mechanical strengths and durability thereof and of the electrostatic characteristics thereof, a serious problem arises with respect to the formation of abnormal images. Abnormal images are often formed when moistened printing or copying paper is used. Such paper will cause deterioration of a resin of the photoconductor by oxidation and deposition of fouling matters on surfaces thereof. As a result, the electric resistance of the surfaces thereof decreases to cause deformation of images.
To cope with the above problem, the following techniques have been proposed.
(1) Japanese Laid-Open Patent Publications Nos. 11-311876 and 2000-131855 disclose a photoconductor having a surface layer formed of a mixed resin containing high and low molecular weight polymers as a binder. While surface fouling matters may be removed by abrasion of the low molecular weight resin, the durability of the photoconductor is not satisfactory.
(2) Japanese Laid-Open Patent Publications Nos. 5-119488, 8-95278 and 2000-214618 disclose a photoconductor in which an anti-oxidizing agent or a plasticizer is incorporated into a photoconductive layer or a surface layer thereof. Japanese Laid-Open Patent Publications Nos. 10-301303 and 1000-10323 disclose the addition of a hindered amine or hindered phenol in a photoconductive layer. While these method can improve the reduction of formation of abnormal images, another problem such as reduction of mechanical strengths or accumulation of residual potential arises.
(3) Japanese Laid-Open Patent Publication No. 11-249333 proposes the use of a charge transporting material having specific ionization potential for the purpose of preventing formation of abnormal images and occurrence of toner filming. Japanese Laid-Open Patent Publications Nos. 7-295278 and 8-184976 disclose a photoconductor having a surface with improved slippage. Japanese Laid-Open Patent Publication No. 6-75386 discloses incorporation of a silicone resin or a fluorine resin to improve slippage of a photoconductor surface. The use of a lubricating agent is, however, not advantageous from the standpoint of residual potential. Further, a slipping property improving agent is generally not compatible with a binder and, therefore, causes a reduction of mechanical strengths of the layer.
Thus, it is difficult to attain both prevention of the formation of abnormal images and the improvement of durability. Japanese Laid-Open Patent Publication No. 11-202525 discloses an image forming process using a specific charging method and a heating method. Japanese Laid-Open Patent Publication No. 11-19087 discloses an image forming process in which a lubricant is fed to a surface of a photoconductor. These methods, however, require additional devices and are disadvantageous with respect to costs and small-size design and, hence, do not meet with the recent needs.
As having been described in the foregoing, the conventional technology for improving durability of photoconductors can be said either to improve the resistance to wearing or to prevent fouling of the photoconductor surface. Namely, the conventional techniques may attain only specific characteristics of the photoconductors but cannot of and by themselves improve service life thereof. In actual, the currently used electrophotographic photoconductors are regarded as consumption type expendable parts.
It is, therefore, an object of the present invention to provide an electrophotographic photoconductor having excellent durability and affording high grade images without abnormal images in a stable manner even when repeatedly used for a long period of time.
Another object of the present invention is to provide an electrophotographic photoconductor which can retain good electrostatic characteristics, which has improved mechanical strengths, which can withstand severe, charge loaded conditions, which can produce images with high resolution and which can prevent formation of abnormal images without using a heater.
It is a further object of the present invention to provide a method of producing the above photoconductor.
It is yet a further object of the present invention to provide an image forming process, an image forming apparatus and a process cartridge for an image forming apparatus which do not require replacement of a photoconductor for a long period of time, which permit the use of a small-sized photoconductor and the high speed printing and which can produce high grade images even in high volume printing.
In accordance with the present invention, there is provided an electrophotographic photoconductor comprising an electroconductive support, and a photoconductive layer formed on said support and having an outwardly facing surface, said photoconductive layer including a charge transporting material, a charge generating material and an inorganic filler comprising xcex1-alumina, wherein the concentration of the inorganic filler in the photoconductive layer decreases from the outwardly facing surface thereof to the opposite surface thereof
In an electrophotographic process, abrasion of a photoconductor is considered to occur or to be accelerated during the following stages:
(1) Abrasion during cleaning stage:
In an electrophotographic process, toner remaining on a photoconductor surface is generally removed by cleaning with a brush or a blade. In the case of the cleaning blade method, an edge of the blade is brought into pressure contact with the surface of the rotating photoconductor to remove the residual toner therefrom. Such a sliding contact causes abrasion or injury of the photoconductor surface. This sort of abrasion is predominantly mechanical abrasion.
(2) Influence during charging stage:
As described in Japanese Laid-Open Patent Publication No. 10-10767, a photoconductor may undergo discharge dielectric breakdown at a defective portion thereof during charging even when the defect is slight. Such dielectric breakdown is significant when the photoconductor is an organic type which has low withstand voltage. Additionally, discharge may cause deterioration of the resin constituting a surface layer of the photoconductor, resulting in a reduction of abrasion resistance. Thus, upon repeated use, the abrasion increases so that the service life is reduced. Since the discharge occurs more strongly at a region of the surface layer having a small thickness, abraded or injured portions caused by repeated use are apt to be deteriorated and, hence, surface undulation is enhanced. As a consequence, adhesive wear or fatigue wear is accelerated.
(3) Abrasion during developing stage:
In the case of a developing method using a two-component developer composed of a toner and a carrier, a photoconductor is subjected to grinding conditions with the carrier and causes abrasion. Further, additives such as a fluidizing agent contained in the toner are generally hard substances and serve as abrasive for the photoconductor. Additionally, the present inventors have found that part of the toner and carrier are retained on a photoconductor surface even after the cleaning treatment with a cleaning blade and causes abrasion.
Abrasion of the photoconductor due to the developer proceeds continually as if it is always filed or polished. Such abrasion poses serious problems especially when the toner used contains a large amount of hard particles such as silica or is easy to stick on a photoconductor surface.
A toner, inclusive of one-component developer, undergoes repeated deposition on a photoconductor surface and separation therefrom. Adhesion between the toner and the photoconductor surface is not ignorable but may cause abrasion when the toner attached to the photoconductor surface is forced to be separated therefrom.
Thus, in order to improve resistance to abrasion of an electrophotographic photoconductor, it is necessary to consider a countermeasure for the above points (1)-(3). The present inventors have made a study with a view toward improvement of the durability of photoconductors and have arrived at a conclusion that the use of an inorganic filler is most effective. Although not wishing to be bound by the theory, a mechanism of contribution of an inorganic filler to improve the durability of a photoconductor would be as follows.
A mere improvement of mechanical strengths (for example a strength expressed by a multiple of a tensile strength by a strain) is not sufficient to improve abrasion resistance of a photoconductor while maintaining desired electrostatic characteristics thereof. One reason for this would be that a step of charging the photoconductor causes a certain change of the photoconductor surface which accelerates abrasion thereof. When the photoconductor surface is formed only of an organic material, there is a limitation in improving ability to withstand voltage so that deterioration of the photoconductor surface by charging is unavoidable. Accordingly, there is a limitation in improving abrasion resistance. The incorporation of an inorganic filler into the photoconductor is thus considered to contribute to the prevention of deterioration by charging.
The present inventors have found that a charge voltage has a great influence upon abrasion rate of a photoconductor. It has been also found that a mode of charging has an influence upon the degree of damage on the photoconductor. It is thus likely that deterioration of the photoconductor surface by charging may accelerate the abrasion thereof by mechanical stress.
When an inorganic filler is incorporated into the photoconductor, the area of the polymer film exposed on the outwardly facing surface thereof decreased in an amount corresponding to the area of the inorgahic filler exposed on the surface. Accordingly, the degree of deterioration of the polymer film is reduced so that the abrasion rate is lowered.
The inorganic filler in the photoconductor also undergoes abrasion and liberation therefrom during electrophotographic processes. Thus, the abrasion resistance of the filler per se and the compatibility and packing characteristics of the filler with the polymer film are also considered to have an influence upon the abrasion resistance of the photoconductor.
Abrasion of a photoconductor during electrophotographic processes proceeds most significantly in the development stage. When the photoconductor surface is formed only of organic materials, the surface hardness thereof is much lower than that of the materials contained in a developer. Thus, incorporation of an inorganic filler, which has a hardness comparable to the materials contained in the developer, into the photoconductor surface will prevent the abrasion thereof by the developer. In addition, the inorganic toner can prevent the polymer on the photoconductor surface from catching the toner and can, thus, contribute to the prevention of abrasion by deposition of toner.
The present inventors have thus found that the prevention of deterioration of a photoconductor surface by charging can improve the abrasion resistance thereof and have investigated various formulation of photoconductor surfaces applicable to various charging modes. As a result, the following findings have been obtained.
(1) Among various inorganic fillers, xcex1-alumina exhibits high abrasion resistance and can improve the abrasion resistance of a photoconductor;
(2) Higher the filler content, the better becomes durability of the photoconductor. A filler content of at least 10% by weight based on a total weight of the photoconductive layer gives satisfactory abrasion resistance;
(3) The use of a binder having a weight average molecular weight of 4.0xc3x97104 or more in the filler-containing layer is effective to immobilize the filler and to improve abrasion resistance thereof;
(4) The large the thickness of a filler-containing protective layer, the better becomes durability of the photoconductor.
With regard to the electrostatic characteristics of photoconductors, the conventional proposals to incorporate an inorganic filler thereinto are not fully satisfactory. In particular, the conventional photoconductors cause a reduction of image contrast due to an increase of electric potential in a light-exposed surface. The present inventors have obtained the following findings as a result of studies with a view toward reducing the electric potential of light-exposed surfaces of photoconductors.
(1) When a filler contained in a photoconductor surface can impart light transmissivity thereto, an increase of the electric potential thereof upon being exposed to light is small. xcex1-Alumina which has high abrasion resistance can improve the light transmissivity and is very effective;
(2) When a filler-containing layer further contains a charge transporting material or a charge generating material in a high concentration, an increase of the electric potential when exposed to light can be made small;
(3) An increase of the electric potential when exposed to light can be generally made smaller by incorporating a filler in a protective layer provided over a photoconductive layer as compared with by incorporating a filler uniformly in the photoconductive layer;
(4) When a filler-free photoconductive layer or charge transporting layer is overlaid with a filler-containing photoconductive layer or charge transporting layer, an increase of the electric potential when exposed to light can be made small. Such functional separation of the photoconductive layer or charge transporting layer can permit an increase of the filler content and of the thickness thereof;
(5) The ratio L/M of the thickness (L) of a filler-containing photoconductive layer to the thickness (M) of a filler-free photoconductive layer is desirably 0.0125-1 for reasons of ensuring good electrostatic characteristics. When the L/M ratio exceeds 1, accumulation of residual potential is generally not ignorable. Too small a L/M ratio of less than 0.0125 generally tends to cause a case where effect of improving durability is not significant. Similarly, the ratio N/P of the thickness (N) of a filler-containing charge transporting layer to the thickness (N) of a filler-free charge transporting layer is desirably 0.0125-0.67 for reasons of ensuring good electrostatic characteristics;
(6) Addition of an electric resistance reducing agent to a filler-containing layer can suppress an increase of the electric potential at a time of light exposure;
(7) A treatment of a filler to impart hydrophobicity can reduce the electric potential at a time of light exposure;
(8) Use of two or more charge transporting materials in combination may reduce the electric potential at a time of light exposure. In addition to improvement of the electrostatic characteristics, gas resistance, mechanical strengths and anti-cracking property may be improved by such a use;
(9) When two or more charge transporting materials are incorporated into a filler-containing or filler-free charge transporting layer and when a difference in ionization potential between them is 0.15 eV or less, an increase of the electric potential when exposed to light can be generally made small. When the difference is greater than 0.15 eV, the residual potential generally increases;
(10) When a difference in ionization potential between a charge transporting material contained in a filler-containing charge transporting layer and a charge transporting material contained in a filler-free charge transporting layer is 0.15 eV or less, an increase of the electric potential when exposed to light can be generally made small. When the difference is greater than 0.15 eV, the residual potential generally increases.
When a photoconductor surface has no light transmissivity, the surface can block light used for writing an image so that the charge generation may be insufficient. In such a case, the electric potential in the electrophotographic apparatus (e.g. electric potential at exposing section and residual electric potential) increases and, therefore, the thickness of the surface layer cannot be increased. In particular, when the light transmittance of the surface layer is less than 15% with respect to the light used for recording, the electric potential in the electrophotographic apparatus tends to increase.
When a filler is incorporated into a protective layer or a photoconductive layer, reflection, refraction and diffusion of incident light occur. Thus, it is desirable that the filler used be small in reflection and refraction. The use of xcex1-alumina is advantageous in this regard, too.
When a charge transporting material and/or a charge generating material are contained in a surface protective layer in a large amount, the protective layer serves to act as a functioning layer showing photoconducting characteristics and can reduce electric potential when exposed to light. By imparting photoconductivity comparable to the conventional photoconductive layer to the surface protective layer, the thickness of the surface layer can be increased. Since the larger the amount of a filler contained in a photoconductive layer, the better becomes the abrasion resistance, it is possible to control the abrasion rate of the photoconductive layer to a desired level by control the thickness thereof and the amount of the filler contained therein. As a consequence, it becomes possible to prevent the occurrence of abnormal images by control of the abrasion rate.
When an electric resistance reducing agent is added to a filler-containing photoconductive layer to accelerate non-trapping of charge carriers or when a surface-modified filler is incorporated into a photoconductive layer to prevent trapping, it is possible to reduce electric potential thereof at a time of light exposure. The charge transporting material to be incorporated into a filler-containing photoconductive layer is desired to show high degree of charge mobility, particularly even in a low electric field region.
It is preferred that a difference in ionization potential between a charge transporting material contained in a filler-containing charge transporting layer and a charge transporting material contained in a filler-free charge transporting layer be small. When the difference in electric potential is large, the electric potential at the time of light exposure tends to increase. Probably, the charge transporting materials in the filler-containing and filler-free charge transporting layers diffuse into respective layers so that the charges are trapped thereby. For the same reason, it is preferred that a difference in ionization potential between two charge transporting materials incorporated into a filler-containing or filler-free charge transporting layer be small.
Next, prevention of a reduction of image resolution will be briefly described.
Abnormal images tend to appear when moistened paper is used. Such paper will cause deterioration of a resin of the photoconductor by oxidation and deposition of fouling matters on surfaces thereof. As a result, the electric resistance of the surfaces thereof decreases to cause deformation of images. It has been found that the use of xcex1-alumina in a photoconductive layer can solve the formation of such abnormal images. Probably, xcex1-alumina is low in degree of absorption of moisture contained in receiving papers.